Photoimageable compositions having improved flexibility and stripping ability

ABSTRACT

A negative-acting photoimageable composition useful as a primary imaging resist in the manufacture of printed circuit boards comprises an acid-functional binder polymer, a photopolymerizable component, and a photoinitiator chemical system, in which all or a portion of the photopolymerizable component comprises a (meth)acrylate functional urethane oligomer, wherein the (meth)acrylate functionality is separated from the urethane linkage by at least two alkylene oxide groups and at least one ring-opened lactone group for improved flexibility, tenting strength, fine line adhesion, developer scumming, coupled with unexpectedly improved resistance to processing chemicals, such as alkaline developing solutions, acid etching solutions, and acid plating baths, and stripping ability in strong alkaline aqueous stripping solutions.

This application is a C-I-P of prior U.S. application Ser. No.09/209,353, filed Dec. 11, 1998.

FIELD OF THE INVENTION

The present invention is directed to negative-acting photoimageablecompositions such as those used as photoresists in the art of printedcircuitry. The photoimageable composition contains, as a portion of itsphotoimageable component, a (meth)acrylate-functional urethane oligomer,wherein the (meth)acrylate functionality is separated from the urethanelinkage by at least two alkylene oxide groups and at least onering-opened lactone group.

BACKGROUND OF THE INVENTION

This invention is directed to negative-acting photoimageablecompositions which are developable in alkaline aqueous solutions. Theinvention is particularly applicable to primary photoimaging resists,but is applicable, as well, to compositions that are hardenable so as toform solder masks and the like.

A variety of such photoimageable compositions are described throughoutthe patent literature. Essential components of the type ofphotoimageable compositions to which the present invention is directedinclude I) a binder polymer, II) photopolymerizable α,β-ethylenicallyunsaturated compound(s), and III) a photoinitiator chemical system. Thebinder polymer I) has sufficient acid functionality, generallycarboxylic acid functionality, that the binder polymer is soluble indilute alkaline aqueous solution and thereby renders the photoimageablecomposition developable in such alkaline aqueous solutions. Thephotopolymerizable compound(s) II) are monomers and/or short chainoligomers, a substantial portion of which have multiple α,β-ethylenicunsaturated functionality.

The photoinitiator chemical system Ill) includes chemicals whichgenerate free radicals upon exposure to actinic radiation. These freeradicals propagate the polymerization of the α,β-ethylenic unsaturatedmoieties of the photopolymerizable compounds II). Herein, thephotoinitiator system III) is deemed to include not only chemicalcompounds which generate free radicals, but catalysts or sensitizerswhich promote the free-radical initiated polymerization of theα,β-ethylenic unsaturated moieties of the photopolymerizable compoundsII).

Printed circuit boards almost invariably have through-holes to establishconnections with circuitry on opposite faces of the board. Photoresistsare required to “tent” these through-holes during processing. With holesbecoming larger on circuit boards, higher tenting strength is becomingincreasingly important; thus greater flexibility of photoimageablecompositions after development is required. Improved flexibility alsocontributes to improved cross hatch adhesion which allows for bettercompatibility with automated polyester support film removal systems usedto separate a support film from the photoresist after exposure andbefore development. If the photoresist is brittle, these support filmremoval systems will cause chipping of the exposed areas of photoresistpredominantly at the edges of the panel and subsequently, circuit linedefects.

By replacing a portion of conventional photoreactive monomers (likeethoxylated trimethylolpropane triacrylate) with an isocyanuric,urethane-based oligomer, a significant improvement to tenting strengthand flexibility was observed. However, even though the flexibility wasnoticeably better, the fine line adhesion was not improved and theoligomer was shown to be a major source of developer scumming.

Improved flexibility, fine line adhesion and lower developer scumminghas been demonstrated when the isocyanuric, urethane-based oligomer iscomprised of the product of a polyethoxymono(meth)acrylate and theisocyanurate trimer of hexamethylene diisocyanate, as described, forexample, in U.S. Pat. No. 5,744,282. The use of a(meth)acrylate-functional urethane product formed from a mono- orpolyalkoxymono(meth)acrylolyl ester, such that the (meth)acrylatefunctionality is separated from the urethane linkage by one or moreflexible alkylene oxide groups, in UV-curable photoresists enhances theperformance of such compositions over those made with urethane compoundsbased on the isocyanurate trimer of hexamethylene diisocyanate.Alternatively, urethane oligomers have been proposed that are formedfrom monoalkoxymono- or di-caprolactone(meth)acrylolyl esters, whichadds a mono- or di-caprolactone chain extension between themonoalkoxy(meth)acrylate functionality and the urethane linkage. Presentday commercial applications require further improvements to flexibility,fine line adhesion and developer scumming, while not interfering withthe chemical resistance of the photoresist to processing solutions andits stripping ability after formation of the patterned copper circuitlines.

Herein, novel (meth)acrylate-functional urethane oligomers based onpolyalkoxy/polylactone (meth)acrylolyl esters are incorporated as atleast a portion of the photopolymerizable component II). The(meth)acrylate-functional urethane oligomers of this invention are foundto significantly improve flexibility and fine line adhesion of thephotoresist and minimize developer scumming. Along with improving theaforesaid properties, it has been found that (meth)acrylate-functionalurethane oligomers further enhance the chemical resistance of theexposed photoresist to processing solutions, such as developing, platingand etching solutions, and also its stripping ability in strong alkalineaqueous solutions.

SUMMARY OF THE INVENTION

The present invention is directed to a negative-acting photoimageablecomposition comprises I) between about 30 and about 80 wt %, based ontotal weight of I) plus II) plus III) of an organic polymeric binderhaving sufficient acid functionality to render the photoimageablecomposition developable in alkaline aqueous solution, II) between about20 and about 70 wt % based on total weight of I) plus II) plus III) ofaddition-polymerizable, non-gaseous α,β-ethylenically unsaturatedcompound(s) capable of forming a high polymer by free-radical initiatedchain-propagating addition polymerization, and III) between about 0.1and about 20 wt % based on the total weight of I) plus II) plus III) ofan organic radiation-sensitive free-radical gene rating photoinitiatorchemical system activatable by actinic radiation to initiatechain-propagating addition polymerization of the addition-polymerizablematerial.

In accordance with the invention, the improvement is wherein componentII) of the photoimageable comprises between about 1 wt % and about 100wt %, preferably at least about 20 wt % up to about 60 wt %, based onthe total weight of II) of a novel (meth)acrylate-functional urethaneoligomer represented by the formula:

T—I—(—P—I—)₉—T

where

I at each independent occurrence is selected from a polyfunctionalaliphatic, cycloaliphatic, or aromatic isocyanate radical having anisocyanate functionality of 2 or greater, preferably an isocyanateradical of the formula:

 or oligomeric, biuret, or isocyanurate thereof,

where R is a polyvalent aliphatic, cycloaliphatic, or aromatichydrocarbon group, with hexamethylene, cyclohexylene, and phenylenebeing generally preferred,

T is bonded to each of the 2 or more isocyanate functionalities presentin I not bonded to P and at each independent occurrence is selected froma (meth)acrylate-functional organic radical of the formula:

 where

R¹ is a hydrogen atom or a methyl group,

A, B and E are in the order given or in any order, preferably in theorder given

A is an alkylene oxide group of the formula:

—[—(CH₂)_(n)—O—]—, or aromatic-substituted derivative thereof,

where n is an integer from 1 to 20, preferably 2 to 4, linear, branched,or cyclic, and,

x is an integer from 1 to 40,

B is an alkylene oxide group of the formula:

—[—(CH₂)_(n1)—O—]—, or aromatic-substituted derivative thereof,

where n₁ is an integer from 1 to 20, preferably 2 to 4, linear,branched, or cyclic, and y is an integer from 0 to 40,

with the alkylene oxide group of B being different from that of A, and Aplus B being formed from at least 2 alkylene oxide groups, preferablyfrom 4 to 12 alkylene oxide groups, and

E is a lactone group of the formula:

where n₂ is an integer from 1 to 20, preferably 3 to 5, linear,branched, or cyclic, or a lactam group of the formula:

where n₂ is defined above and R² is defined below, although a lactonegroup is generally preferred, and

z is an integer from 1 to 40, preferably 3 to 8, and

m is an integer from 1 to 40, with m typically being 1, and

P is selected from any polyfunctional alcohol group, thiol group, aminegroup or phosphate group, with a polyfunctional alcohol group beinggenerally preferred; and,

q is 0 or an integer from 1 to 10.

In the above, if q is 1 or more, P is even more preferably apolyfunctional alcohol group represented by the formula:

—[—O—G—]—

where G is of the formula:

—(A)_(s1)—(B)_(s2)—(E)_(s3)—(D)_(0 or 1)—(W)_(0 or 1)—(J)_(0 or 1)—(B)_(s4)—(A)_(s5)—

where A, B, E and R are defined above,

D is an diester functional alkoxy radical of the formula:

where t is an integer from 1 to 40,

W is a radical of the formula

where V is an acidic group selected from —COOH, —SO₃H, and PO₃HR² whereR² is a hydrogen atom or a C₁₋₁₈ alkyl radical,

J is an ester functional alkyl radical of the formula:

where t₁ is an integer from 1 to 6, and,

with the proviso that if D+W+J=0, then Σs₁ . . . s₅ must be≧1.

The use of the above (meth)acrylate-functional urethane oligomer for allor part of the photopolymerizable component II) of the photoimageablecomposition, wherein the (meth)acrylate functionality in the oligomer isseparated from the urethane linkage by a flexible block copolymer of atleast two alkylene oxide groups and at least one ring-opened lactonegroup, solves the problems of the prior art and enables thephotoimageable composition formed therefrom to possess excellentmechanical strength, flexibility, fine line adhesion, and resistance todeveloper scumming, combined with excellent chemical resistance toprocessing solutions and stripping ability.

The present invention also provides a photoimageable element comprisinga support film and a layer of said photoimageable composition formedthereon.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

Herein, unless otherwise noted, all percentages are weight percentages.Component I) (the binder polymer) and Component II) (the photoimageablecompounds(s)) and Component III) (the photoinitiator chemical system)are herein considered to equal 100 wt %, and other components arecalculated as parts relative to 100 parts of I) plus II) plus III).

The term “(meth)acrylate” is meant to encompass “acrylate” and“methacrylate” functionality and mixtures thereof.

The invention is directed to photoimageable compositions which aredevelopable in alkaline aqueous solution and which therefore havesubstantial acid functionality. Such photoimageable compositionstypically have a binder polymer I) having acid functionality, typicallyan acid number of at least about 80, preferably at least about 100 andmore preferably about 150 or more, up to about 250. The acidfunctionality is typically carboxylic acid functionality, but may alsoinclude, for example, sulfonic acid functionality or phosphoric acidfunctionality. Binder polymers for photoimageable compositions typicallyhave weight average molecular weights between about 20,000 and about200,000, preferably at least about 80,000.

The polymers are typically derived from a mixture of acid functionalmonomers and non-acid functional monomers. Some specific examples ofsuitable acid functional monomers are acrylic acid, methacrylic acid,maleic acid, fumaric acid, citraconic acid,2-acrylamido-2-methylpropanesulfonic acid, 2-hydroxyethyl acrylolylphosphate, 2-hydroxypropyl acrylol phosphate, 2-hydroxy-alpha-acryloylphosphate, etc. One or more of such acid functional monomers may be usedto form the binder polymer.

The acid functional monomers may be copolymerized with non-acidfunctional monomers, such as esters of acrylic acid and methacrylicacid, for example, methyl acrylate, 2-ethyl hexyl acrylate, n-butylacrylate, n-hexyl acrylate, methyl methacrylate, hydroxy ethyl acrylate,butyl methacrylate, octyl acrylate, 2-ethoxy ethyl methacrylate, t-butylacrylate, 1,5-pentanediol diacrylate, N,N-diethylaminoethyl acrylate,ethylene glycol diacrylate, 1,3-propanediol diacrylate, decamethyleneglycol diacrylate, decamethylene glycol dimethacrylate,1,4-cyclohexanediol diacrylate, 2,2-dimethylol propane diacrylate,glycerol diacrylate, tripropylene glycol diacrylate, glyceroltriacrylate, 2,2-di(p-hydroxyphenyl)-propane dimethacrylate, triethyleneglycol diacrylate, polyoxyethyl-2-2-di(p-hydroxyphenyl)-propanedimethacrylate, triethylene glycol dimethacrylate,polyoxypropyltrimethylol propane triacrylate, ethylene glycoldimethacrylate, butylene glycol dimethacrylate, 1,3-propanedioldimethacrylate, butylene glycol dimethacrylate, 1,3-propanedioldimethacrylate, 1,2,4-butanetriol trimethacrylate,2,2,4-trimethyl-1,3-pentanediol dimethacrylate, pentaerythritoltrimethacrylate, 1-phenyl ethylene-1,2-dimethacrylate, pentaerythritoltetramethacrylate, trimethylol propane trimethacrylate, 1,5-pentanedioldimethacrylate, and 1,4-benzenediol dimethacrylate; styrene andsubstituted styrene, such as 2-methyl styrene and vinyl toluene andvinyl esters, such as vinyl acrylate and vinyl methacrylate to providethe desired acid number. The amount of the binder polymer I) may varyover a wide range, typically comprising between about 30 to about 80 wt% of the composition based on total weight of I) plus II) plus III).

Examples of such polymers and photoimageable compositions using suchpolymers are found, in the following U.S. Pat. Nos. 3,953,309,4,003,877, 4,610,951, and 4,695,527 the teaching of each of which areincorporated herein by reference.

The photopolymerizable component II) described above havingphotopolymerizable poly α,β-ethylenic unsaturation comprises betweenabout 1 wt % and about 100 wt %, preferably at least about 20 wt % up toabout 60 wt % of total amount of II), of the (meth)acrylate-functionalurethane oligomer described above, i.e., between about 2 wt % and 70 wt%, preferably between about 5 wt % and 45 wt % based on total weight ofI) plus II) plus III).

There are two currently preferred methods for forming the(meth)acrylate-functional urethane oligomers of this invention. Thepreferred method of forming the first group of urethane oligomersdefined in the above formula by q=O is to initially block copolymerizeone or more lactone groups and two or more alkylene oxide groups onto a(meth)acrylic acid backbone by a conventional addition polymerizationprocedure, to produce the (meth)acrylate-functional group of formula Twhich has a hydroxy-terminus opposite the (meth)acrylate functionality.

While in the above described formula for T, (A), (B) if present, and (E)may theoretically be in any order, the preferred mode of oligomersynthesis generally dictates that (E) be at the terminus opposite the(meth)acrylate functionality. In the preferred synthetic route,(meth)acrylic acid is reacted with an alkylene oxide monomer oraromatic-substituted alkylene oxide monomer or a mixture of suchmonomers so as to produce (A). If desired, further reaction is carriedout with an different alkylene oxide monomers, aromatic-substitutedalkylene oxide monomers or mixture of such monomers to produce (B). Theresulting product is then reacted with a lactone or lactam monomer ormixture of lactone or lactam monomers to produce (E).

Alkylene oxide monomers used in forming the (A) and (B) alkylene oxidegroups generally contain 1 to 20 carbon atoms, although short chainalkylene oxides of at least 2 up to 4 carbon atoms, such as ethyleneoxide, propylene oxide, butylene oxide and tetrahydrofuran, withethylene oxide and propylene oxide being preferred. Also,aromatic-substituted alkylene oxide monomers, such as styrene oxide, maybe used to form (A) and (B). While (B), if present, is herein defined asis (A), (B) is formed from a different monomer from (A). For example (A)could be formed from ethylene oxide and (B) could be formed frompropylene oxide, or (A) could be formed from a mixture of ethylene oxideand tetrahydrofuran while (B) could be formed from a mixture ofpropylene oxide and styrene oxide. The optional incorporation of (B)allows the oligomer to be tailored to particular applications. Toprovide sufficient chain length to the oligomer, (A) plus (B) must beformed from at least 2 alkylene oxide monomers total, preferably between4 and 12 monomers.

Lactone component (E) of the (meth)acrylate functional oligomeric groupdefined in above formula T is formed from 1 to 40 lactone monomer units,either a single lactone species or mixture of lactone species. Thelactone species employed generally have from 1 to 20 carbon atoms (notincluding the carbonyl carbon), although 3 to 5 carbon atom species aregenerally preferred. Epsilon-caprolactone is especially preferredlactone for forming (C). Other suitable lactones include, but are notlimited to, beta-butyrolactone, zeta-enantholactone,delta-valerolactone. Also, C₁-C₆ alkyl-substituted lactones, such as thealkyl delta-valerolactones, such as methyl-, ethyl-, hexyl-, dimethyl-,diethyl-, di-n-propyl-, di-n-hexyl-, di-iso-propyl-, trimethyl-,triethyl-, and tri-n-propyl-epsilon caprolactones, as well as C₁-C₆alkoxy- and aromatic-substituted lactones may also be used. In additionto lactones, lactams may be substituted for forming component (E).Suitable lactams include those which correspond to the lactones listedabove, with epsilon-caprolactam being especially preferred.

The oligomer thus formed is hydroxy-terminated at the terminus oppositethe (meth)acrylate functionality. Subsequently, the hydroxy-terminated(meth)acrylate oligomer is reacted with a polyfunctional isocyanate usedin forming I in the above formula by a conventional urethane additionpolymerization procedure, to produce the (meth)acrylate-functionalurethane oligomer component II) of the present invention.

In the urethane reaction, the conditions are chosen so that the hydroxyterminal functionality of the (meth)acrylate functional oligomer reactswith all of the isocyanalte functionalities present in I, to end-capeach of the isocyanate groups with T. In the above formula, only 2 T'sare shown as a matter of convenience. However, it should be understoodthat if I is tri-functional, rather than di-functional, 3 isocyanateend-capping T's will be present instead of 2 T's, and so on.

The polyfunctional isocyanates used in forming I can be a wide varietyof organic isocyanates having a functionality of 2 or greater, oroligomers, biurets, or isocyanurates thereof. In the general formulaabove, simple examples of R groups include hexamethylene, phenylene, andcyclohexylene, although more complex divalent hydrocarbon moieties arealso suitable. The choice of R is generally not considered to beparticularly critical, the selection generally depending upon thecommercial availability of suitable precursors. Examples of suitablepolyisocyanates include 1,6-hexamethylene diisocyanate,1,4-tetramethylene diisocyanate, 1,3- and 1,4-phenylene diisocyanate,4,4′-methylenebis(phenylisocyanate), 2,4-toluene diisocyanate,1,2,4-benzene triisocyanate, 1,4-cyclohexylene diisocyanate, isophoronediisocyanate, 4,4′-methylene bis(cyclohexyl isocyanate), polymethylenepolyphenyl isocyanate, or biurets thereof, such as the biuret ofhexamethylene diisocyanate, or isocyanurates thereof, such as theisocyanurate of hexamethylene diisocyanate, although hexamethylenediisocyanate, its biuret and its isocyanurate are especially preferred.

The preferred method for forming the second group of urethane oligomerswhere q≧1 is to initially react the polyfunctional isocyanate with apolyfunctional alcohol by a conventional addition polymerizationprocedure, to form a polyisocyanate/polyol adduct represented by theformula I—(P—I)_(q). As can be from the formula, the reaction conditionsare chosen so as to form an isocyanate-terminated urethane oligomer tothe virtual exclusion of alcohol-terminated polymeric materials. Theoptional incorporation of the (P—I)_(q) group into the urethane oligomerallows the oligomer to be tailored to particular applications.Subsequently, the isocyanate-terminated adduct is reacted with thehydroxy-terminated (meth)acrylate oligomer T defined above by aconventional urethane addition polymerization procedure, to produce the(meth)acrylate-functional urethane oligomer component II) of the presentinvention.

The polyfunctional alcohol may be selected from a variety of materialsas indicated by the polyfunctional alcohol group defined as —[—O—G—]— inthe above formula. Examples of suitable polyfunctional alcohols include:monomeric or polymeric diols, such as ethylene glycol, propylene glycol,1,4-butanediol, 2-ethyl-1,6-hexanediol, 1,10-decanediol,1,4-bis-hydroxymethylcyclohexane, diethylene glycol, triethylene glycol,polyethylene glycols having molecular weights (Mw) from about 200 to1,500, the reaction products of 4,4′-dihydroxydiphenyl ether,4,4′-dihydroxydiphenyl sulfide, 4,4′-dihydroxydiphenyl methane,4,4′-dihydroxydiphenylpropane or 4,4′-dihydroxydiphenylsulfone with 0 to40 moles of alkylene oxide, dihydroxycycloalkyls,bishydroxydicycloalkyls, polypropylene glycols, polytetrahydrofuran,polybutylene glycols, thioethylene glycol and dithiotriethylene glycol;polyester polyols, such as polycaprolactone, polybutyrolactone,polyethylene terephthalate, polypropylene adipate, polybutylene adipate,polyethylenebutylene sebacate, and other polyester polyols havingmolecular weights (Mw) in the range of about 500 to 3,000. Apart fromthe diols, monomer or polymeric compounds having 2 to 6 aliphatichydroxy groups, such as glycerol, trimethylolpropane, pentaerythritol,dipentaerythritol, sorbitol, or polyalkoxylate derivatives of these;polymeric polyester polyols including the lactone polyesters; blockcopolymers of polyethers and polyesters having terminal hydroxy groups;and, caprolactone polyols and polysiloxane polyols, or the like. Otherpolyfunctional alcohols which contain acidic groups may also be used,including those described in U.S. Pat. No. 5,415,972. In addition topolyfunctional alcohols, polyfunctional amine, thiol or phosphatecompounds may be substituted. For instance, examples of suitablepolyfunctional amines include 1,6-hexanediamine, isophoronediamine,diaminodiphenylmethane, polyalkoxydiamines, etc. Examples ofpolyfunctional thiols include 1,2-ethanedithiol, 1,4-butanedithiol,1,6-hexanedithiol, 1,9-nonanedithiol, etc. Examples of polyfunctionalphosphates include any of a variety of available diphosphates.

Through use of the above-described (meth)acrylate-functional urethaneoligomier whereby the chain which links the (meth)acrylate functionalityto the urethane group is extended with at least two alkylene oxidegroups and at least one ring-opened lactone or lactam group, improvedflexibility and tenting strength of the cross-linked system is achieved.The flexibility is achieved by incorporation of a long chain attached tothe crosslinkable ethylenically unsaturated (meth)acrylatefunctionality. Coupled with improved flexibility, the urethane oligomerimproves the adhesive properties of the resist to the copper cladsurface of blank circuit board following lamination. Better adhesionenables the production of a fine line (less than 75 microns) resistsidewall that adheres better to the copper surface of the circuit board.

Most surprising is the improvement seen in stripping and chemicalresistance to processing solutions. Because the urethane oligomerproduces better adhesion, stripping the resist from the copper surfacewould be expected to be more difficult. While not wishing to be bound byany particular theory, it is believed that by distancing the(meth)acrylate functionality from the urethane block with not onlyflexible alkylene oxide groups but also durable and high modulus lactoneor lactam groups, the ester links present in the ring-opened lactone orlactam portion provide sites for hydroxide attack during the strippingoperation, thereby greatly shortening stripping time. While producingsites for stripping solution attack, the relatively hydrophobic chainextension also provides good chemical resistance to alkaline developingsolution, acid plating baths and acid etching solutions.

The balance of photopolymerizeable component II), if any, is typically amonomer, dimer or short chain oligomer having ethylenic unsaturation,particularly, α,β-ethylenic unsaturation, including monofunctionalcompounds and compounds having α,β-ethylenic unsaturation functionality2 or greater. Typically, a mixture of mono-functional andmulti-functional monomers will be used. Suitable photopolymerizeablecompounds include, but are not limited to, the monomers recited above assuitable for forming binder polymers, particularly the non-acidfunctional compounds. Other particularly useful photopolymerizeablecompounds are styrene maleic anhydride copolymers, or similaranhydride-containing copolymers, partially esterified withhydroxy-functional (meth)acrylic esters such as hydroxyethyl acrylate,polyethoxymono(meth)acrylate, and polyalkoxypolylactonemono(meth)acrylates such as those described herein and characterized byT. Polymers of the latter type are further described in and further setforth in U.S. Patent Application of Lundy, Barr and Reardon, entitled“Photoimageable Compositions Having Improved Chemical Resistance andStripping Ability”, filed the same day herewith, the teachings of whichare incorporated herein by reference.

The total amount of photopolymerizeable component II) is typicallybetween about 20 and about 70 wt % of the photoimageable compositionbased on total weight of I) plus II) plus III).

To initiate polymerization of the monomers upon exposure to actinicradiation, the photoimageable composition contains a photoinitiatorchemical system. Generally, the photoinitiator chemical system comprisesbetween about 0.1 and about 20 wt % based on total weight of I) plus II)plus III). Suitable photoinitiator chemicals include, but are notlimited to, 9-phenylacridine, n-phenyl glycine, aromatic ketones(benzophenone, N, N′-tetramethyl-4, 4′-diaminobenzophenone [Michler'sketone],N,N′-tetraethyl-4,4′-diaminobenzophenone,4-methoxy-4′-dimethylaminobenzophenone,3,3′-dimethyl-4-methoxybenzophenone,p,p′-bis(dimethylamino)benzophenone,p,p′-bis(diethylamino)-benzophenone, anthraquinone,2-ethylanthraquinone, naphthaquinone, phenanthraquinone), benzoins(benzoin, benzoinmethylether, benzoinethylether, benzoinisopropylether,benzoin-n-butylether, benzoin-phenylether, methylbenzoin, ethybenzoin,etc.), benzyl derivatives (dibenzyl, benzyldiphenyldisulfide,benzyldimethylketal (SIC), etc.), acridine derivatives(9-phenylacridine, 1,7-bis(9-acridinyl)heptane, etc.), thioxanthones(2-chlorothioxanthone, 2-methylthioxanthone, 2,4-diethylthioxanthone,2,4-dimethylthioxanthone, 2-isopropylthioxanthone, etc.), acetophenones(1,1-dichloroacetophenone, p-t-butyldichloroacetophenone,2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone,2,2-dichloro-4-phenoxyacetophenone, etc.), 2,4,5-triarylimidazole dimers(e.g., 2-(o-chlorophenyl)-4,5-diphenylimidazole dimer,2-(o-chlorophenyl)-4,5-di(m-methoxyphenyl imidazole dimer,2-(o-fluorophenyl)-4,5-diphenylimidazole dimer,2-(o-methoxyphenyl)-4,5-diphenylimidazole dimer,2-(p-methoxyphenyl)-4,5-diphenylimidazole dimer,2,4-di(p-methoxyphenyl)-5-phenylimidazole dimer,2-(2,4-dimethoxyphenyl)-4,5-diphenylimidazole dimer,2-(p-methylmercaptophenyl)-4,5-diphenylimidazole dimer, etc.) etc.Though, not a free-radical generator, triphenylphosphine may be includedin the photoinitiator chemical system as a catalyst.

The photoimageable composition may advantageously include one or moireplasticizers at between about 0.5 and about 10 wt % relative to thetotal weight of I) plus II) plus III). Examples of suitable plasticizersinclude, but are not limited to, phthalate esters (e.g.,dibutylphthalate, diheptylphthalate, dioctylphthalate,diallylphthalate), glycols (e.g., polyethylene-glycol,polypropyleneglycol), glycol esters (e.g., triethylene-glycoldiacetate,tetraethyleneglycoldiacetate, dipropyleneglycol-dibenzoate), phosphateesters (tricresylphosphate, tripheynlphosphate), amides(p-toluenesulfoneamide, benzenesulfoneamide, -n-butylacetoneamide),aliphatic dibasic acid esters (diisobutyl-adipate, dioctyladipate,dimethylsebacate, dioctylazelate, dibutylmalate, triethylcitrate,tributylcitrate, triethylacetylcitrate, tri-n-propylacetylcitrate,tri-n-butylacetylcitrate, butyl-laurate,dioctyl-4,5-diepoxycyclohexane-1,2-dicarboxylate,glycerinetriacetylesters, dipropyleneglycol dibenzoate,polyethyleneglycol 200 dibenzoate, sucrose benzoate, trioctyltrimellitate, etc.

Compositions of the present invention typically include a color formerto provide contrast to the light-exposed photoimageable composition.Color formers are typically used at between about 0.1 and about 1.0 wt %relative to total weight of I) plus II) plus III). Suitable colorformers include, but are not limited to, diphenylamine, dibenzylaniline,triphenylamine, diethylaniline, diphenyl-p-phenylenediamine,p-toluidine, 4, 4′-biphenyldiamine, o-chloroaniline, etc., leuco crystalviolet, leuco malachite green, etc.

Additionally, the photoimageable compositions may contain a wide varietyof additional components as are known in the art, including additionalpolymers, such as those which might be used to effect a final hardenedcure of a solder mask, dyes, color formers, stabilizers, flexibilizingagents, fillers, etc.

Processing of the photoimageable composition is in a conventionalmanner. In a typical procedure, a photoimageable composition layer,either formed from a liquid composition or as transferred as a layerfrom a dry film, is applied to a copper surface of a copper-clad board.When a dry film is used, the dry film typically comprises a liquidphotoimageable composition dried onto a flexible polyester supportsheet, e.g., polyethylene terephthalate, which is preferablytransparent. A protective sheet, e.g., polyethylene, is usually providedon the surface of the dried photoimageable layer opposite the supportsheet before the film is rolled into reels. The protective sheet isremoved prior to application, e.g., lamination, to the copper-cladboard. Once applied to the board (with the photoimageable compositionfacing the board), the photoimageable composition is then exposed toactinic radiation through appropriate artwork. Exposure to actinicradiation polymerizes the monomer in the light-exposed areas, resultingin a cross-linked structure that is resistant to developer. Next, thesupport film is removed from the top of the photoimageable compositionand the exposed photoimageable layer is then is developed in dilutealkaline aqueous solution, such as a 1% sodium carbonate solution. Thealkali solution causes salt formation with the carboxylic groups of thebinder polymers, rendering them soluble and removable. Afterdevelopment, an etchant may be used to remove copper from those areaswhere the resist was removed, thereby forming a printed circuit. Afterdevelopment, an alternative process would be to build up the thicknessof the exposed copper areas using an electrolytic plating procedure.After either etching or plating, the remaining resist is then removedusing an appropriate stripper, such as 3% sodium hydroxide solution.

This invention is applicable to photoresists used in both etching andelectrolytic plating processes. A particular advantage is enhancedresistance to process solutions coupled with rapid removal in strippingsolutions which is truly a unique combination of properties not normallyencountered with highly chemically resistant photoresists.

The invention will now be described in greater detail by way of specificexamples.

EXAMPLE 1 Preparation of Urethane Oligomers Comprising PolyfunctionalIsocyanates End-Capped with Poly(alkoxylated)/Poly(lactone) BlockCopolymer Mono(meth)acrylates

Urethane oligomers of the general structure T—I—(—P—I—)_(q)— T where q=0of this example were prepared by the following procedure. A reactionvessel, fitted with a stirrer, thermocouple, and temperature bath, ischarged with a poly(alkoxylated)/poly(lactone) mono(meth)acrylatealcohol (derived from polylactone additions to a polalkoxylatedmono(meth)acrylate alcohol), antioxidants, and catalyst. Thereafter, thepolyisocyanate is slowly added to the agitated alcohol mixture under adry air blanket at an NCO/OH ratio of about 0.9/1.0. The temperature iscontrolled throughout the addition process and usually is held at30°-35° C. After all of the isocyanate has been added, the mixture isheld at 35°±5° C. for 1-4 hours, then cooled to ambient conditions andpackaged.

The antioxidants used in these synthesis are of the phenol types and areeither hydroquinone monomethyl ether and/or octadecyl3,5-bis(1,1-dimethylethyl)-4-hydroxybenzene-propanoate, but selectiondoes not have to be limited to these two materials.

The catalysts used in these synthesis can be of any of the usualorganometallics commonly utilized in urethane reactions, but in all ofthe examples cited has been dibutyltin dilaurate.

Examples of urethane oligomers prepared by this method include:

Compound 1

H₂C═C(CH₃)CO—O—(—CH₂—CH₂—O—)_(x)—(—CO—CH₂—CH₂—CH₂—CH₂—CH₂—O—)_(z)—CO—NH—

(CH₂)₆—NH—CO—(—O—CH₂—CH₂—CH₂—CH₂—CH₂—CO—)_(z)—(—O—CH₂—CH₂—)_(x)—O—CO—C(CH₃)═CH₂,

where in T: for (A), x=5-6, for (B), y=0, and for (E), z=3,4, or 6.

Compoung 2

where T is:

H₂C═C(CH₃)CO—O—(—CH₂—CH₂—O—)_(x)—(—CO—CH₂—CH₂—CH₂—CH₂—CH₂—O—)_(z)—

and for (A), x=5-6, for (B), y=0, and for (E), z=3, 4, or 6.

EXAMPLE 2 Preparation of Urethane Oligomers Comprising PolyfunctionalIsocyanate Prepolymers End-Capped with Poly(alkoxylated)/Poly(lactone)Block Copolymer Mono(meth)acrylates

Urethane oligomers of the general structure T—I—(—P—I—)_(q)— T where q≧1of this example were prepared by the following procedure. First, areaction vessel, fitted with a stirrer, thermocouple, and temperaturebath is charged with a polyisocyanate and catalyst. Thereafter, apolyfunctional alcohol (hereafter referred to as a polyol) is slowlyadded under dry nitrogen to the agitated isocyanate mixture (preheatedto about 40° C.) at a NCO/OH ratio of about 2.04/1.0, to produce anisocyanate-terminated polyol adduct. The mixture temperature is allowedto elevate (from the exotherm) to approximately 60° C., and thencontrolled to keep the exothermic rise from exceeding 60° C. until allof the polyol has been added and the exotherm has peaked. Thetemperature is then maintained at anywhere from 60° C. to 80° C.,depending on the selection of polyol and polyisocyanate, for 1 to 2hours, and then is cooled to below 40° C.

In a second reaction vessel, also fitted with a stirrer, thermocouple,and temperature bath, is charged a poly(alkoxylated)/poly(lactone)mono(meth)acrylate alcohol(meth)acrylate monoalcohol and antioxidant.Under a dry air blanket, the polyisocyanate reaction product from thefirst vessel is slowly added to the agitated (meth)acrylate alcoholmixture, at an NCO/OH ratio of 1.0/1.0, keeping the temperature at30°-35° C. After all of the polyisocyanate reaction product had beenadded, the temperature is held at 35°±5° C. for 1 to 4 hours, then iscooled to ambient and packaged.

Examples of the urethane oligomers which have been prepared by thismethod include:

In all cases

T=H₂C═C(CH₃)CO—O—(—CH₂—CH₂—O—)₅₋₆—(—CO—CH₂—CH₂—CH₂—CH₂CH₂—O—)₄—and for(B), y=0

and,

I=—CO—NH—(CH₂)₆—NH—CO—

Compound 3

P follows the structure —[—O—G—]— where G=—(A)₅—(W) —(A)₅— andW=bisphenol A, or in other words,

P=—(—O—CH₂—CH₂—)₅—(Bisphenol A)—(—CH₂—CH₂—O—)₅—

and,

q=1

Compound 4

P follows the structure —[—O—G—]— where G=—(A)₃—(W)—(A)₃— andW=bisphenol A, or in other words,

P=—(—O—CH₂—CH₂—)₃—(Bisphenol A)—(—CH₂—CH₂—O—)₃—

and,

q=1

Compound 5

P follows the structure —[—O—G—]— where G=—(A)₇—, or in other words,

P=—O—CH(CH₃)—CH₂—[—O—CH₂—CH(CH₃)—]₆—O—

and,

q=1

Compound 6

P follows the structure —[—O—G—]— where G=—(A)₁₃—, or in other words,

P=—O—CH(CH₃)—CH₂—[—O—CH₂—CH(CH₃)—]₁₂—O—

and,

q=1

Compound 7

P follows the structure —[—O—G—]— where G=—(A)₁₇—, or in other words,

P=—O—CH(CH₃)—CH₂—[—O—CH₂—CH(CH₃)—]₁₆—O—

and,

q=1

Compound 8

Follows the structure T—I—(—P—I—)₃—T or T—I—P—I—P′—I—P—I—T

P follows the structure —[—O—G—]— where G=—(A)₅ —(W)—(A)₅— andW=bisphenol A, or in other words,

P=—(—O—CH₂—CH₂—)₅—(Bisphenol A)—(—CH₂—CH₂—O—)₅—

and,

q=1

P′ follows the structure —[—O—G—]— where G=W=

where V=COOH, or in other words

Compound 9

Is like Compound 8 except that

P follows the structure —[—O—G—]— where G=—(A)₇—, or in other words,

P=—O—CH(CH₃)—CH₂—[—O—CH₂—CH(CH₃)—]₆—O—and,

q=1

Compound 10

Is like Compound 3 except that

P follows the structure —[—O—G—]— where G=J and for J, R=C₁₈H₃₄ andt₁=˜3, or

in other words,

P is Castor Oil

Compound 11

Is like Compound 10 except that

P follows the structure —[—O—G—]— where G=J—A₅, and for J, R=C₁₈H₃₄ andt₁=˜3, or in other words,

P is ethoxylated Castor Oil (15 moles EO)

EXAMPLE 3 Comparison of Negative-Acting Photoresists

The following ingredients were blended together in the given proportionsto provide a negative-acting photoresist composition of the presentinvention (Formulation 2) along with a comparative photoresistcomposition (Formulation 1).

Wt % Ingredients 1 2 Acrylic Binder Polymer¹ 39 39 Acrylic BinderPolymer² 12 12 Urethane Oligomer A³ 15 Urethane Oligomer B⁴ 15Polyethoxylated BisphenolA 14 14 Dimethacrylate Polpropylene Glycol 1111 Monomethacrylate Aromatoc Sulfonamide 4.5 4.5 Aromatic Acridine 0.10.1 Photoinitiator Leuco Dye 0.35 0.35 Bis Dialkylaminoketone 0.05 0.05Halogenated Iophine Dimer 3.5 3.5 Poylacrylate Rheology 0.1 0.1 ModifierAromatic Polyacid 0.05 0.05 Polyaromatic Phosphine 0.3 0.3Triphenylmethane Dye 0.05 0.05 Table Footnotes ¹90,000 Mw copolymer ofMMA, MAA, n-BA, Tg 90° C., 150 acid number. ²25,000 Mw copolymer ofstyrene, AA, Tg 102° C., 239 acid number. ³Comparative Urethane Oligomerof the Structure:

where for T R¹ = —CH₃, and for A, n = 2 and x = 6, while for B, y = 0and for E, z = 0 and q = 0 ⁴Urethane Oligomer of the Present Inventionof the Structure:

where for T R¹ = —CH₃, and for A, n = 2 and x = 6, while for B, y = 0and for E, n = 5 (linear) and z = 4 and q = 0.

Each mixture was prepared at about 55% solids in 2-butanone and coatedonto a 0.8 mil polyester carrier film and dried to approximately 1%residual VOC's. A thin film of about 1.5 mils thickness was obtained.The films were then laminated at 121° C., 40 psi, 1 meter per minute,onto chemically cleaned 1 oz. copper/0.059 FR-4/1 oz. clad copperlaminates and imaged on a 5 kw printer through a silver halide phototoolwith an adjusted exposure to obtain a copper step of 7 and 9 as measuredwith a Stouffer®21 step wedge. The imaged panels were then developed in1% sodium carbonate monohydrate at 49° C. to remove the photoresist inthe unexposed portions, followed by several spray rinses using tap waterand the deionized water. Using an etching process to gather some of thedata, the developed panels were then etched in 2N cupric chloride/HClsolution at 45° C. The panels were then stripped of the imaged anddeveloped photoresist in a 3% sodium hydroxide solution at 49° C.,followed by a spray rinse of tap water. Using an electrolytic platingprocess for the remainder of the data, another set of developed andrinsed panels having a suitable plating pattern imaged thereon weredipped in a liquid aid preplate cleaner (LAC 81 marketed by MortonElectronic Materials) for about 3 minutes, then rinsed with tap waterfor 1 minute, followed by etching for 30 seconds in a 10% strengthammonium peroxysulfate solution. The panels were then rinsed again withtap water, immersed for 1 minute in 10% strength fluoroboric acid, andthen electroplated for 30 minutes in fluoboric tin/lead plating solution(current density 15 amps/ft², metal deposit approximately 30μm at roomtemperature), followed by a 5 minute rinse with tap water.

Performance test results are given in the Table below.

Performance Formulations Tests 1 2 Thickness (μm) 37 μm 36 μm BreakPoint¹ 14.5 sec 18.4 sec (1% Na₂CO₃.H₂O, 29.4° C.) Sensitivity (mJ) 2376 solid step 7 Sensitivity (mJ) 45 149 solid step 9 Stripping Time², 39sec 34 sec step 7 (3% NaOH, 54° C.) Stripping Mode³, sm-lg Xlg step 7(3% NaOH, 54° C.) Stripping Time, 40 sec 34 sec step 9 (3% NaOH, 54° C.)Stripping Mode, vsm-md mdlg-Xlg step 9 (3% NaOH, 54° C.) Fine Line Adh30 μm 30 μm (step 7, 2 × B.P.) Resolution 50 μm 55 μm (step 7, 2 × B.P.)Solid SST41 22 21.7 (step 7, 2 × B.P.) Fine Line Adh 45 μm 35 μm (step7, 4 × B.P.) Resolution⁴ 45 μm 40 μm (step 7, 4 × B.P.) Solid SST41 20.819.7 (step 7, 4 × B.P.) Fine Line Adh 25 μm 25 μm (step 9, 2 × B.P.)Resolution 70 μm 80 μm (step 9, 2 × B.P.) Solid SST41 28.0 28.1 (step 9,2 × B.P.) Tent Strength⁵, wet 584 gms 947 gms (20° C., step 7, 6.0 mm)Tent Strength, wet 643 gms 1088 gms (20° C., step 9, 6.0 mm) Fine LineAdh, μm 70 40 (step 7 after etch) Fine Line Adh, μm 50 37 (step 9 afteretch) Cross Hatch Adh 98 100 % intact (after dev., step 7) Cross HatchAdh 75 90 % intact (after dev., step 9) Underplating (after slight veryslight plating 38 μm Sn/Pb) Tape Adhesion poor very good (after Sn/Pbplating) Stripping Time at 101 sec 80 sec step 7 (after Sn/Pb plating)Stripping Time at >180 sec 92 sec step 9 (after Sn/Pb plating) TableFootnotes ¹The breakpoint time was recorded at the point the resistdissolved completely in 1% Na₂CO₃.H₂O at 29.4° C. ²The stripping timewas recorded at the point the resist stripped completely in 3% NaOH at54° C. ³The particle size of the stripped resist pieces coming off theboard was visually recorded. ⁴The resolution is measured as the minimumvalue of fully developed equal lines and spaces. ⁵Puncture strength with2 mm radius tipped probe through a tented 6 mm hole after 4 minuteimmersion in 20° C. tap water.

From the foregoing it will be seen that this invention is one welladapted to attain all ends and objects hereinabove set forth togetherwith the other advantages which are apparent and inherent. Since manypossible variations may be made of the invention without departing fromthe scope thereof, the invention is not intended to be limited to theembodiments and examples disclosed, which are considered to be purelyexemplary. Accordingly, reference should be made to the appended claimsto assess the true spirit arid scope of the invention, in whichexclusive rights are claimed.

What is claimed:
 1. A negative-acting photoimageable compositioncomprising: I) a binder polymer having sufficient acid functionality torender said photoimageable composition developable in alkaline aqueoussolution; II) an addition-polymerizable non-gaseous α,β-ethylenicallyunsaturated compound(s) capable of forming a high polymer byfree-radical initiated chain-propagating addition polymerization; andIII) a radiation-sensitive free-radical generating photoinitiatorchemical system activatable by actinic radiation to initiatechain-propagating addition polymerization of the addition-polymerizablematerial, wherein all or a portion of said component II) comprises a(meth)acrylate functional urethane oligomer having the formula:T—I—(—P′—I—)_(q)—T where I at each independent occurrence is a radicalof the formula:

where R is a polyvalent aliphatic, cycloaliphatic, or aromatichydrocarbon group; T at each independent occurrence is selected from a(meth)acrylate-functional radical of the formula:

where R¹ is a hydrogen atom or a methyl group, A, B and E are in theorder given or in any order, A is an alkylene oxide group of theformula: —[—(CH₂)_(n)—O—]— where n is an integer from 1 to 20 and x isan integer from 1 to 40, B is an alkylene oxide group of the formula:—[—(CH₂)_(n1)—O—]— where n1 is an integer from 1 to 20 and y is aninteger from 0 to 40 with the alkylene oxide group of B being differentfrom that of A, and A plus B being formed from at least 2 alkylene oxidegroups, and E is a ring-opened lactam group of the formula:

where n2 is an integer from 1 to 20 and R² is a hydrogen atom or a C₁₋₁₈alkyl radical, or E is a mixture of the ring-opened lactam group and aring-opened lactone group of formula:

where n2 is as defined above and z is an integer from 1 to 40; P is ofthe formula: —[O—G—]— where G is of the formula:—(A)_(s1)—(B)_(s2)—(E)_(s3)—(D)_(f)—(W)_(k)—(J)_(u)—(B)_(s4)—(A)_(s5)—where A, B, E and R are defined above, f is 0 or 1, k is 0 or 1 and u is0 or 1, D is a diester functional alkoxy radical of the formula:

where t is an integer from 1 to 40, W is a radical of the formula:

where V is an acidic group selected from —COOH, —SO₃H, and PO₃HR² whereR² is a hydrogen atom or a C₁₋₁₈ alkyl radical, J is an ester functionalalkyl radical of the formula:

where t1 is an integer from 1 to 6, and, with the proviso that iff+k+u=0, then Σs₁ . . . s₅ must be ≧1.
 2. The photoimageable compositionof claim 1, wherein said (meth)acrylate functional urethane oligomercomprises between about 5 wt % and about 45 wt % based on total weightof I) plus II) plus III).
 3. The photoimageable composition of claim 1,comprising: I) between about 30 and about 70 wt % based on total weightof I) plus II) plus III) of said binder polymer; II) Between about 20and about 70 wt % based on total weight of I) plus II) plus III) of saidaddition-polymerizable, non-gaseous α, β-ethylenically unsaturatedcompound(s); and III) Between about 0.1 and about 20 wt % based on thetotal weight of I) plus II) plus III) of said photoinitiator chemicalsystem.
 4. The photoimageable composition of claim 3, wherein said(meth)acrylate functional urethane oligomer comprises between about 1and 100 wt % of II).
 5. A dry film photoresist useful in the manufactureof printed circuit boards, comprising: a polymeric support sheet and alayer of the photoimageable composition of claim 1 thereon.
 6. Thephotoimageable composition of claim 1, wherein q=0.
 7. Thephotoimageable composition of claim 1, wherein q=1 to
 10. 8. Anegative-acting photoimageable composition comprising: I) a binderpolymer having sufficient acid functionality to render saidphotoimageable composition developable in alkaline aqueous solution; II)an addition-polymerizable non-gaseous α,β-ethylenically unsaturatedcompound(s) capable of forming a high polymer by free-radical initiatedchain-propagating addition polymerization; and III) aradiation-sensitive free-radical generating photoinitiator chemicalsystem activatable by actinic radiation to initiate chain-propagatingaddition polymerization of the addition-polymerization material, whereinall or a portion of said component II) comprises a (meth)acrylatefunctional urethane oligomer having the formula:

T is:H₂C═C(CH₃)CO—O—(—CH₂—CH₂—O—)_(x)—(—CO—CH₂—CH₂—CH₂—CH₂—CH₂—O—)_(z)—,Wherein x=5 or 6 and z=3, 4, or 6.